LAMELLIBRANCHIATA. 



217 



In these cases it is necessary that there should be orifices in 

 the mantle-sac by which water can be admitted to the gills, and 

 can be expelled again from the body. The margins or lips of 

 these orifices are usually drawn out or extended into longer or 

 shorter muscular tubes, which are termed the " siphons," and 

 which may be either separate, or may be united to one another 

 along one side. The Bivalves which possess these siphons 

 are said to be " siphonate," and there are two leading modi- 

 fications in the arrangement of these tubes. In the Siphonate 

 Bivalves which spend their existence buried in sand or mud, 

 as well as in many other cases, the siphons are long, and can 

 be partially or entirely retracted within the shell by means of 

 special muscles, called the " retractor-muscles of the siphons." 

 In these cases, the pallial line does not run in an unbroken 

 curve, but is deflected inwards posteriorly, so as to form an 

 indentation or bay, which is termed the " jDallial sinus " (fig. 

 1 68, 2). The presence, therefore, of an indented pallial line 

 shows that the animal possessed retractile siphons. In other 



Fig. 1 68. Shells of Lamellibranchiata. i. Cyclas amnica, a dimyary shell with an 

 entire pallial line. 2. Tapes pttllastra, a dimyary shell with an indented pallial line. 

 3. Feriia. ephippiuni, a monomyary shell. (After Woodward.) a Pallial line; b 

 Muscular impressions left by the adductors ; c Siphonal impression. 



Bivalves the respiratory siphons are of small size, and are des- 

 titute of retractor muscles, so that they cannot be withdrawn 

 within the shell. In these cases the " pallial line," or the im- 

 pression caused by the attachment of the muscular border of 

 the mantle, is unbroken in its curvature, and presents no 

 indentation (fig. 168, i). In another group of the Bivalves 

 there are no respiratory siphons at all, and the mantle-lobes 

 are free, and are not united to one another at their edges. In 

 these cases also, the pallial line is unbroken or " simple." 

 When, therefore, we find a Bivalve shell in which the pallial 

 line is not indented by a sinus, we know that the animal which 

 inhabited the shell either possessed no siphons, or that if siphons 

 were present, they were small and not retractile. 



